1. Field of the Invention
The present invention relates to an energy absorbing intermediate shaft which is built into a steering system of an automobile in order to transmit the movement of a steering wheel to a steering gear.
2. Related Background Art
In order to transmit the movement of a steering wheel to a steering gear, a steering system for an automobile has the constitution shown in FIG. 9. As shown in FIG. 9, a steering wheel 2 is fixed to the upper end of a steering shaft 1, and a steering column 3 is fixed under an instrument panel 6 with an upper bracket 4 and a lower bracket 5. Said steering shaft 1 is rotatably set through the steering column 3. The lower end of the steering shaft 1 which protrudes from the lower open end of the steering column is connected via a first universal joint 7 with the upper end of an intermediate shaft 8. Further, the lower end of the intermediate shaft 8 is connected via a second universal joint 9 with the input shaft 10 of a steering gear (not shown).
In the above-mentioned constitution, the rotation of the steering wheel 2 is transmitted through the steering shaft 1 inserted through the steering column 3, the first universal joint 7, the intermediate shaft 8, the second universal joint 9, the input shaft 10, and finally, to the steering gear, thereby giving a steering angle to wheels.
Generally, in the steering system having the constitution described above, the steering column 3, the steering shaft 1 and the intermediate shaft 8 are of the energy absorbing type which, when given an impact, absorb the energy of the impact and shorten their overall lengths in order to protect the driver at the time of a car crash. For example, Japanese Laid-Open Patent Appln. No. 3-79472 discloses one of the typical energy absorbing intermediate shafts.
This typical energy absorbing intermediate shaft disclosed the above Laid-Open Patent consists of four components which are assembled together, as shown in FIG. 10. One end (the left end in FIG. 10) of a shaft 11 and a yoke 12 which constitutes a universal joint are welded together. On the periphery near the other end of the shaft 11, male splines 13 are formed. Two concave grooves 14a and 14b are formed around the periphery on which said male splines 13 are formed.
Female splines 16 are formed on the inner periphery of a connection tube 15, which has three through holes 17a, 17b and 17c penetrating from the inner periphery to the outer periphery of the connecting tube 15. These through holes are separately aligned in the axial direction, wherein the distance between the through holes 17a and 17b is the same as the distance between the above-mentioned grooves 14a and 14b.
On the periphery of a connecting shaft 18, male splines 19 are formed. Two grooves 14c and 14d are formed around the periphery on which the male splines 19 are formed.
One end of a tube 20 is an open end, while the other end thereof and a yoke 21 which constitutes another universal joint are welded together. The tube 20 also has a through hole 17d penetrating from the inner periphery to the outer periphery of the tube 20. The distance between the through hole 17d and the above-mentioned through hole 17c when said connecting tube 15 attaches against the tube 20 is the same as the distance between the grooves 14c and 14d formed around the periphery of the connecting shaft 18. Further the depth D from the open end of the tube 20 to the end face of the yoke 21 is the same as the length L of the connecting shaft 18 (D=L).
These components 11, 15, 18 and 20 formed as described above are assembled together as shown in FIG. 11, and are fixedly set to each other by injecting synthetic resin 22a-22d through the through holes 17a-17d into the grooves 14a and 14d and then curing the synthetic resin. As the synthetic resin 22a-22d injected through respective through hole 17a-17d fills the entire grooves 14a-14d and cures therein, the components 11, 15, 18 and 20 are fixedly set to each other so that they can neither rotate around the axis nor shift in the axial direction. Note that a clearance A between the right end face of the shaft 11 and the left end face of the connecting shaft 18 and a clearance B between the right end face of the connecting shaft 18 and the end face of the yoke 21 are formed when the components 11, 15, 18 and 20 are assembled together as described above. The connecting shaft 18 which connects the connecting tube 15 with the tube 20 not only transmits torque between the tubes 15 and 20 but also prevents the contact position of the tubes 15 and 20 from bending.
When a car which has the intermediate shaft 8 constructed as shown in FIG. 11 built in its steering system crashes and the front thereof is collapsed, the second adjustable joint 9 including said yoke 12 is pushed backwards (rightwards in FIG. 11), thereby giving large compressive force in the axial direction to the intermediate shaft 8. By this compressive force, first, the synthetic resins 22a and 22b bonding the shaft 11 and the connecting tube 15 are torn and broken. Then, the shaft 11 shifts backwards by the length of said clearance A and the right end face of the shaft 11 comes in contact with the left end face of the connecting shaft 18.
When the shaft 11 in the above state is further pushed backwards, the synthetic resins 22c and 22d bonding the connecting shaft 18 to the connecting tube 15 and the tube 20 are torn and broken. Then, the connecting tube 15 relatively shifts by the length of said clearance B and the right end face of the connecting shaft 18 and the end face of the yoke 21 come in contact with each other. In this state, the open end of the tube 20 and the left end face of the connecting shaft 18 come in the same plane, and bearing capacity of the connecting shaft 18 which connects the tube 20 with the connecting tube 15, (that is, which prevents the contact portion of the tubes 20 and 15 from bending) disappears.
As a result, the shaft 11 and the connecting shaft slip off and separate from each other. After that, the shaft 11 does not push backwards the connecting shaft 18 nor the tube 20. Accordingly, even if the second universal joint 9 is further pushed backwards at the time of the car crash, the first universal joint 7 is not pushed backwards. Thus, as the steering wheel 2 does not protrude toward the driver, it cannot hurt the driver.
As for the above-mentioned typical energy absorbing intermediate shaft, however, since the four separately prepared components 11, 15, 18 and 20 are first assembled together at predetermined positions and then are fixedly set to each other with synthetic resin 22a-22d, the assembly operation is difficult and the manufacturing cost necessarily rises.
In addition, since two separately prepared shaft members, that is, the shaft 11 and the connecting shaft 18, constitute the shaft, at least three fixing points should be provided in order to fixedly connect the shafts 11 and 18 with the connecting tube 15 and the tube 20. In order to provide said fixing points, the shafts 11 and 18 should have certain lengths of engagement with the tubes 15 and 20. In short, when more fixing force is required, the overall length of the intermediate shaft 8 becomes longer. In such a case, troubles arise. For example, it becomes difficult to arrange the intermediate shaft 8 between the first and second universal joints and obtain sufficient clearances for collapse inside the intermediate shaft 8.